1. Field of the Invention
This invention relates to a method of making an optical fiber bundle having flexibility, and the optical fiber bundle produced by the method. More particularly, this invention relates to a method of making an optical fiber bundle wherein individual optical fibers are firmly fixed together at least at a portion of the optical fiber bundle, and are separated from one another at a portion other than the aforesaid fixed portion so that the optical fiber bundle has flexibility the invention is also in the optical fiber bundle produced by the method.
2. Description of the Prior Art
Optical fiber bundles are used for image guides, light guides and the like. When an optical fiber bundle is used as an image guide, it is necessary to correctly arrange the individual fibers so that they are maintained in identical geometrical patterns at both end portions in order to obtain a sharp and correct image. Furthermore, for example when the optical fiber bundle for the image guide is used in an endoscope, the middle portion between the end portions of the optical fiber bundle must be flexible and the end portions must be capable of being bent freely and acutely so that the endoscope can be inserted into any desired portion in a hollow body, such as human body cavity, an interior space in a machine, or the like, in order to observe the portion concerned.
Various methods have been proposed to make an optical fiber bundle wherein the middle portion between the end portions has flexibility and the end portions can be bent freely and acutely.
For example, it is know to make a flexible optical fiber bundle for use as an image guide by introducing a core glass having a high refractive index into an inner crucible of a double crucible and introducing a cladding glass having a low refractive index into the outer crucible, heating the double crucible to an appropriate temperature, drawing both glasses from a hole positioned at the botom of the double crucible so as to cover the core glass with the cladding glass, closely looping the obtained optical fiber in one row, and fixing a portion of the formed loop with an adhesive. Then, another optical fiber is formed and closely looped in one row on the previously formed loop in the same way as described above, and fixed with an adhesive at the fixed section of the previously formed loop. The aforesaid operations are repeated to obtain a loop-like optical fiber bundle having a desired thickness, approximately the center of the fixed section of the loop-like optical fiber bundle is cut perpendicularly to the length of the optical fibers, and the cut faces are ground or polished. In this method, since a very thin optical fiber having a thickness of, for example, about 20.mu. is formed by one heating operation, the subsequent arrangement work requires a high-degree skill in handling an extremely thin optical fiber, and there is risk of the fiber breaking. Thus, this method is disadvantageous in that the image guide can be produced only at a low yield and, consequently, is expensive.
It is also known to make an optical fiber bundle by use of an acid-soluble glass. In this method, a core glass having a high refractive index is introduced into the innermost crucible of a triple crucible, a cladding glass having a low refractive index is introduced into an intermediate crucible, and an acid-soluble glass is introduced into the outermost crucible. The whole triple crucible is heated to an appropriate temperature, and the molten glasses are drawn from a hole positioned at the bottom of the triple crucible so as to cover the core glass with the cladding glass and further cover the surface of the cladding glass with the acid-soluble glass. In this way, a triple optical fiber having a diameter of about 200.mu. is formed and cut to an appropriate length of about 400 mm. Many (e.g. 10,000) fibers thus obtained are bundled, fused together, heated to an appropriate temperature, and stretched until the diameter of each optical fiber is reduced to about 1/15 the initial diameter. Both end portions of the hard optical fiber bundle thus obtained are covered with an acid resistant covering material, and then the whole optical fiber bundle is contacted with an acid (for example, nitric acid) to dissolve out the acid-soluble glass from the middle portion of each optical fiber. In this method, since a relatively thick fiber having a thickness of about 200.mu. is handled, the optical fiber arranging work is easier than in the first-mentioned method, and there is less risk of the fiber's breaking. Further, since the fibers are fused together with heat after they are arranged, there is no risk of the fused fibers breaking. Accordingly, this method can produce an optical fiber bundle with a higher yield and at a lower cost than is possible with the first-mentioned method.
However, in the second method mentioned above there is a large fluctuation in the acid dissolution rate during acid treatment. For example, in some hard optical fiber bundles, the acid-soluble glass covering the respective fibers is relatively quickly dissolved out from the fibers at the middle portions of the optical fiber bundles, and the fibers at the middle portions become separated from one another. However, in other hard optical fiber bundles, it sometimes happens that the acid-soluble glass is dissolved out only from the fibers near the peripheries of the optical fiber bundles, and the fibers positioned at the centers of the optical fiber bundles become surrounded by protective layers which inhibit the acid solution from permeating into interstices among individual fibers at the centers of the optical fiber bundles to dissolve the acid-soluble glass therefrom. It is presumed that, since the protective layers mainly comprise silica gel, dissolution with the acid solution becomes difficult. When large fluctuation occurs in the acid dissolution rate from on optical fiber bundle to another, the cladding layers of the individual optical fibers of the optical fiber bundles exhibiting a higher acid dissolution rate suffer from erosion by the acid solution over a longer period, and the mechanical strength of these optical fiber bundles becomes low. In order to eliminate the above-mentioned drawback, various attempts have been made to contact the surfaces of the hard optical fiber with a fresh acid solution by subjecting the acid solution to ultrasonic wave treatment, moving the hard optical fiber bundles in the acid solution, or agitating the acid solution. However, due to breakage or entanglement of the optical fibers, these attempts have not been successful in providing flexible optical fiber bundles of high quality at a high efficiency. Further, it has been proposed in Japanese Patent Publication No. 56(1981)-27841 to employ an apparatus wherein a hard optical fiber bundle is inserted into a pipe or a space defined by partition plates, both end portions of the optical fiber bundle are fixed by clamps of the pipe or the partition plates, and an acid solution is introduced into the pipe or the space defined by the partition plates to dissolve the acid-soluble glass from the fibers at the middle portion of the optical fiber bundle. However, in this apparatus, in order to minimize fluctuation in the acid dissolution rate in treatment with the acid solution, it is necessary to greatly increase the flow rate of the acid solution. Consequently, there arises a risk of the optical fibers breaking or being entangled.
Besides the fluctuation in the acid dissolution rate, the aforesaid method presents the very real problem that an insoluble residue of glass remains unremoved at portions near the border lines between the flexible middle portion of the optical fiber bundle and the end portions thereof where the individual fibers are fixed to one another after treatment with the acid solution, and the life of the product is adversely affected by the insoluble residue. In order to eliminate this drawback, various method have been proposed. For example, Japanese Patent Publication No. 56(1981)-47526 and U.S. Pat. No. 4,086,045 disclose an optical fiber bundle having end portions reinforced by covering the portions near the interfaces between the flexible middle portion of the optical fiber bundle and the end portions thereof, where an insoluble residue of glass remains unremoved, by a plastic exhibiting appropriate hardness and appropriate flexibility, and further charging said plastic into the interstices among individual fibers at portions close to the interfaces. Japanese Unexamined Patent Publication No. 50(1975)-98344 discloses an optical fiber bundle having high strength against bending wherein the portions near the interfaces between the flexible middle portion of the optical fiber bundle and the end portions thereof, where an insoluble residue of glass remains unremoved, is covered by a reinforcing pipe the flexibility of which varies in a continuous or step-wise gradient. Japanese Patent Publication No. 56(1981)- 48844 describes an optical fiber bundle having end portions reinforced by impregnating the interstices among individual fibers at the semi-dissolved portions (where an insoluble residue of glass exists) between the flexible middle portion of the optical fiber bundle and the solid end portions thereof with a high-molecular material exhibiting high hardness, good adhesion and low viscosity, curing the high-molecular material, impregnating the vicinities of the portions where the high-molecular material bleeds with an elastomer exhibiting low viscosity, and curing the elastomer. Japanese Patent Publication No. 53(1978)-24815 and U.S. Pat. No. 3,624,816 describe a method of making an optical fiber bundle having flexibility by employing a silica-free glass as the acid-soluble glass and substantially completely removing the insoluble residue of glass from the optical fiber bundle. U.S. Pat. No. 3,383,192 discloses a method of charging a plastic into the vicinities of the interfaces between the flexible middle portion of an optical fiber bundle and the end portions where the respective fibers are fused together with heat.
However, the prior techniques mentioned above have various drawbacks as described below. Namely, in the optical fiber bundles disclosed in Japanese Patent Publication No. 56(1981)-47526, U.S. Pat. No. 4,080,045, Japanese Unexamined Patent Publication No. 50(1975)-98344 and Japanese Patent Publication No. 56(1981)-48844, the portions where an insoluble residue of glass remains unremoved cause the lengths of the end portions of the optical fiber bundle to become longer and make is difficult to acutely bend the end portions. Further, when a plastic is charged into interstices among individual fibers at portions close to the interfaces between the middle portion and the end portions of the optical fiber bundle to reinforce the portions close to the border lines, it is not always possible to completely mix the plastic with the insoluble residue of glass among the optical fibers at the portions close to the interfaces. Also, from the viewpoint of the chemical composition, size and amount of the insoluble residue, or the like, it is not always possible for the insoluble residue to attain the same effect as fillers employed in fiber-reinforced plastics (FRP). Conversely, the presence of the insoluble residue of glass presents a risk of damage to the optical fibers. Further, in the techniques disclosed in the four publications mentioned above, it is not always possible to efficiently make an optical fiber bundle for use as an image guide exhibiting high mechanical strength and uniform quality because of fluctuation in the acid dissolution rate.
In the optical fiber bundles disclosed in Japanese Patent Publication No. 53(1978)-24815 and U.S. Pat. No. 3,624,816, since a silica-free glass is employed as the acid-soluble glass, substantially no insoluble residue of glass remains in the optical fiber bundles. This is very convenient in the process of treatment with an acid solution. In general, however, since silica-free glass exhibits very large changes in viscosity with changes in temperature, it is very difficult to form fibers having an extremely high dimensional accuracy such as the optical fibers of an image guide should have. Also in the method disclosed in U.S. Pat. No. 3,383,192, no insoluble residue remains unremoved since only the fibers at the end portions of the optical fiber bundle are fused together with heat and treatment with an acid solution is not carried out. However, since the end portions where the individual fibers are fixed to one another and the vicinities of the end portions are heated to a temperature above the annealing temperature of the glass, the mechanical strength and the flexibility of the optical fiber bundle becomes very low in the cases of thin optical fibers (having a thickness of, for example, about 15.mu.) such as fibers employed in an image guide. In general, it is known that the mechanical strength of glass fibers considerably decreases even with heating at a temperature below the softening temperature.